In 3GPP (Third Generation Partnership Project), in a radio communication system corresponding to LTE (Long term Evolution), for which the standards are being set up currently, a radio base station eNB assigns a radio resource in the radio communication between the radio base station eNB and a radio terminal UE (for example, see Non Patent Literature 1). Furthermore, in a radio communication system corresponding to LTE, either Frequency Division Duplex (FDD) or Time Division Duplex (TDD) is employed in the radio communication between the radio base station eNB and the radio terminal UE. Moreover, in an LTE (TDD-LTE) radio communication system employing the TDD, there has been discussed a feature where a radio base station eNB performs control for adaptively directing a beam (adaptive array control) toward the radio terminal UE at the time of transmission of a downlink radio signal, in order to ensure communication quality between the radio base station eNB and a radio terminal UE that is moving.
According to a technique of calculating the antenna weight, when a radio base station eNB receives a sounding reference signal (SRS), which is an uplink radio signal from a radio terminal UE, it is considered that the radio base station eNB calculates an antenna weight for a downlink resource block that has a frequency band equal to a frequency band of the SRS. In such a case, the radio base station eNB notifies the frequency band (SRS transmission frequency band) used in the transmission of SRS to the radio terminal UE.
However, because the radio terminal UE exists on an outer edge of a cell, in cases where the power consumption increases when SRS is transmitted by using an entire broad SRS transmission frequency band, the SRS is sometimes transmitted by using only a part of the SRS transmission frequency band. Under such a situation, the radio base station eNB cannot accurately set the antenna weight for a downlink resource block in the entire notified SRS transmission frequency band.
Furthermore, because the SRS transmission frequency band is wide, the radio terminal UE may exceed the upper limit of the transmission power. Under such a situation, the radio terminal UE is forced to reduce the transmission power, the certainty of arrival of SRS to the radio base station eNB deteriorates.
In view of the above problem, a first object of the present invention is to provide a radio base station and a communication control method, by which it is possible to accurately set an antenna weight for a downlink radio resource. Furthermore, a second object of the present invention is to provide a radio terminal and a communication control method by which the certainty of arrival of a reference signal to a radio base station is improved.